Radiation hardened and other high reliability electronic circuits are desired for a variety of applications in which systems and circuits are exposed to radiation, electromagnetic interference (EMI) or other adverse electrical noise conditions. Example applications include satellites and other spacecraft, aircraft, medical devices such as x-ray equipment, circuits used in nuclear power plants, processor cores and other sensitive digital circuits. In such applications, radiation can cause latchup in metal oxide semiconductor (MOS) circuits due to wells and doped regions that operate as bipolar transistors. During such a latchup condition, these parasitic bipolar transistors can be turned on by current flow beneath the MOS circuit components, leading to potentially large currents that interfere with operation of logic circuits in an integrated circuit (IC), and can sometimes cause the IC to become permanently damaged. Latchup typically involves inadvertent creation of a low-impedance path between the power supply rails of a MOSFET circuit, triggering a parasitic PNPN (silicon controlled rectifier or SCR) structure operating as a PNP and an NPN transistor stacked next to each other. During a latchup when one of the parasitic bipolar transistors is conducting, the other conducts and both keep each other in saturation as long as the structure remains forward-biased with some non-zero current flow. A single event latchup (SEL) is a latchup caused by a single event upset, typically from heavy ions or protons from cosmic rays or solar flares. Various integrated circuit applications require circuitry that operates in such radiation environments, and MOS circuit latchup can lead to processor circuits freezing up, requiring restarting or power cycling. Accordingly, improved semiconductor devices and fabrication techniques are desired for circuit applications involving radiation exposure to address digital circuit latchup problems.